It is known that, in the fabrication of several types of steel the incorporation of the nitrogen(N) leads to the formation of stable nitrides with elements such as chromium(Cr), vanadium(V), niobium(Nb), aluminium(Al), etc., improving the austenitic structure of steel.
It is also known that, in certain cases, the nitrogen can replace, in some proportion, the nickel(Ni), due to its powerful gammagene properties.
In the iron crystal-structure, the nitrogen takes an interstitial position, which creates a distortion of the structure and contributes to the decrease in the size of the crystal-structure and the increase in the resistance to grain growth in working.
The nitrogen bearing steels, especially the stainless steels with chromium, are more ductile, more workable over a wider range of chromium content. Cold forming operations may be carried out more succesfully over a wider range of temperatures.
Further, the tensile strength of commercial steels is much improved by the presence of nitrogen. A small proportion of nitrogen (i.e. 0.008%) can improve the yield point and thus replace a certain quantity of manganese(Mn), which disturbs the rimming process in the big moulds (over 15 tons ingots). This is the reason for the tendency to alloy the rimmed steel with nitrogen.
Regarding the art of introducing nitrogen into the liquid steel, the following methods should be mentioned:
The introduction of molecular nitrogen in liquid steel through nozzles or perforated electrodes, below the slag layer and its conversion into atomic state, which is incorporated by the liquid steel, PA1 The blowing of compressed air or nitrogen-gas through the lance of the converter, together with oxygen, or the blowing of molecular nitrogen or ammonia(NH3) in the ladle through porous refractories, PA1 The introduction of organic substances containing nitrogen into the liquid steel (in furnace or ladle), PA1 The introduction of nitrogen bearing refined ferro-alloys (ferro-manganese or ferro-chromium) into the liquid steel (in furnace or ladle). PA1 metallic ejections (grits, sludges, powder from exhausted fumes-flue dust) resulting from various steel making processes, PA1 iron oxides (powder and scale), steel powder and iron powder resulting from secondary steel processes such as forging, rolling, drawing, etc., PA1 iron pellets and their residual powder, PA1 iron powder.
With the exception of the ferro-alloy method, all the other methods have a limited field of application or a nitrogen assimilation with a high degree of dispersion.
The ferro-alloy method is used more often because it facilitates the nitrogen penetration into the liquid steel in a wide range of nitrogen content and with a small scattering of the assimilated nitrogen.
However, due to the manganese or chromium content in the final analysis of the steel, there is a limit to the amount of nitrogen which can be introduced, because together with the nitrogen important quantities of manganese or chromium are assimilated into the steel (10-15 times more than nitrogen). These refined ferro-alloys bear high contents of manganese or chromium and therefore are more expensive than the standard ferro-alloys.
This invention discloses a new type of ferro-alloy, the FERRO-NITROGEN or NITRIDED IRON, which bears an important content of nitrogen but is free of other elements such as manganese or chromium.
The FERRO-NITROGEN keeps the advantages of the conventional ferro-alloys method but is not limited in its usage by the presence of manganese or chromium. The FERRO-NITROGEN can be used in the fabrication of any type of steel in which the presence of nitrogen is necessary for the achievement of all the improvements the nitrogen conveys to the physical properties of the steel.